1. Technical Field
This disclosure relates generally to an image forming apparatus, and more particularly, to an image forming apparatus using a recording head including a liquid ejection head that ejects liquid droplets, and a method for correcting landing positions of liquid droplets.
2. Description of the Background
One example of related-art image forming apparatuses such as printers, copiers, plotters, facsimile machines, and multifunction devices having two or more of printing, copying, plotting, and facsimile functions is an inkjet recording device employing a liquid ejection recording method. The inkjet recording device includes a recording head that ejects droplets of a recording liquid such as ink onto a sheet of a recording medium while the sheet is conveyed to form an image on the sheet.
Examples of the inkjet recording device include a serial-type image forming apparatus, in which the recording head ejects liquid droplets while moving in a main scanning direction to form an image on the sheet as the sheet is moved in a sub-scanning direction perpendicular to the main scanning direction, and a line-type image forming apparatus equipped with a line-type recording head that ejects liquid droplets and does so without moving to form an image on the sheet as the sheet is moved in the sub-scanning direction.
A maintenance mechanism that maintains performance of the recording head is essential for the image forming apparatus employing the liquid ejection recording method. One of the functions of the maintenance mechanism is to discharge bubbles, foreign substances, coagulated ink, and so forth present in the recording head through nozzles in the recording head in order to prevent irregular ejection of the ink from the nozzles in the recording head.
In addition, a full-color image forming apparatus that forms full-color images using the liquid ejection recording method generally includes two separate recording heads, that is, a recording head that ejects black ink droplets (hereinafter referred to as the first recording head) and a recording head that ejects color ink droplets (hereinafter referred to as the second recording head). In such a full-color image forming apparatus, not only black ink but also color ink is ejected for maintenance of the recording heads even when monochrome printing is performed using only the first recording head, causing a waste of color ink and a concomitant cost increase.
In order to solve those problems, some image forming apparatuses deploy separate carriages for the black and color inks. That is, they include a first carriage mounting a first recording head that ejects black ink droplets and a second carriage mounting a second recording head that ejects color ink droplets. The first and second carriages are separatably dockable with each other.
Meanwhile, there are also image forming apparatuses that correct a timing of ejection of ink droplets from recording heads (hereinafter referred to as an ejection timing) in order to prevent a shift in positions on a sheet to which black and color ink droplets are ejected (hereinafter referred to as landing positions of ink droplets). Specifically, the image forming apparatus forms an adjustment pattern and reads the adjustment pattern using an optical sensor to adjust the ejection timing of the black and color ink droplets, thereby correcting the landing positions of the black and color ink droplets on the sheet and reducing color shift during full-color image formation.
However, because the carriage mounting the recording head scans reciprocally to form images on the sheet for each of outward and homeward scanning movement, a shift in the landing positions of the ink droplets between outward and homeward scanning movement tends to occur especially upon formation of ruled lines in the above-described image forming apparatuses. In addition, in a case in which the first and second carriages respectively mounting the first and second recording heads are docked together to form full-color images, a color shift tends to occur when ink droplets of different colors are superimposed one atop the other to form the full-color images on the sheet.
To solve the above-described problems, an arrangement is often employed in which a test pattern formed on a recording medium or a conveyance belt is read by a sensor installed on a carriage to adjust landing positions of ink droplets ejected from recording heads by, for example, controlling the timing of the ejection of the ink droplets.
It is to be noted that any variation or instability in scanning speed of the carriage adversely affects accuracy in reading of the test pattern using the sensor installed on the carriage. Therefore, it is preferable that the carriage be as heavy as possible to accurately read the test pattern.
In the above-described case in which the two separate carriages dockable with each other are used for full-color image formation, the carriages are docked together to make the carriage having the sensor thereon heavier so that the scanning speed of the carriages is stabilized during reading of the test pattern, thereby accurately reading the test pattern.
However, when the landing positions of the ink droplets are corrected during monochrome image formation, the first carriage needs to be separated from the second carriage, to form monochrome images by scanning only the first carriage. As a result, docking and separation of the first and second carriages must be performed between formation and reading of the test pattern.
In addition, when multiple first recording heads are offset laterally from each other on the first carriage in order to increase productivity during monochrome image formation, the number of times the first and second carriages are separated from and docked with each other for forming and reading the test pattern, respectively, is further increased.
Consequently, a period of time required for docking and separating the carriages with and from each other and moving the carriages to a position where docking and separation of the carriages are performed is increased, thereby extending downtime for adjustment of the landing positions.